Survival of Juvenile Giant Kelp: The Effects of Demographic Factors, Competitors, and Grazers

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Abstract
Patterns of survival of juvenile giant kelp (Macrocystis pyrifera) were examined in relation to grazing by sea urchins, shading by adult kelp, density of recruits, densities of other algal species, and substrate distribution. Survival was poorer in areas where white sea urchins (Lytechinus anamesus) were abundant or where there was an overlying canopy of adults. At other sites, the density of recruits explained the greatest proportion of variability in survival, and the proportion of juveniles that survived was negatively correlated with the number of recruits. Potential algal competitors (Pterygophora californica and Cystoseira osmundacea), red sea urchins (Strongylocentrotus franciscanus), and substrate distributions had no significant effect on survival. The mechanisms of density—dependent survival were similar to "dominance and suppression" models that explain self—thinning in many terrestrial plant populations. In stands where densities of recruits were high, skewed size distributions developed, probably as the result of competition for light. High—density stands had proportionally more small plants than low—density stands, and, during subsequent storms, only the larger plants survived. This led to a more equitable distribution of survivors than would be expected based on the number of recruits. Intraspecific interactions, both between adults and juveniles and among juveniles of the same cohort, appear to be important structuring forces in most giant kelp populations. The parallels in many terrestrial populations. However, in other algal populations, there is little evidence for density—dependent mortality or for the dominance—suppression hypothesis. These differences may relate to competition for light, which is intense in Macrocystis forests and many terrestrial populations, but less so in assemblages of other benthic algae.